= Waveguide
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A <waveguide> confines a wave transversely while allowing propagation along a longitudinal direction. Material properties and <boundary conditions> select transverse <normal modes>, whose longitudinal <dispersion relations> may have cutoffs even when the bulk material is nondispersive. Examples include an <optical fiber>, a hollow <electromagnetic waveguide>, and a <membrane waveguide>. Guiding reduces geometric spreading along the chosen route but does not eliminate absorption, scattering or leakage.
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